One of the most difficult quality control problems in the paper making industry, for example, has been the monitoring of color, brightness, and opacity within specified limits in the course of manufacture of the paper. The color and brightness of a white surface and its opacity greatly influence its appearance to the human eye and the intensity of any color applied to it. As a matter of fact, the interrelationship between color and opacity is so intimate as to make the monitoring of one without the other virtually meaningless.
In the prior art, the problem heretofore has been that of efficiently controlling the three variables of color, opacity and brightness. The only approach available is the use of individual sensors for measuring these variables. Such a system not only is awkward and complicated but also fails to provide consistent quality.
I have invented an on-line system for continuously monitoring the color, opacity and brightness of a moving web, which system overcomes the defects of systems of the prior art. My system permits of the accurate reproduction of any shade and any color. My system permits of self-standardizing for all three variables without removal from the line on which it is used. My system has color, brightness and opacity stability with a sensitivity surpassing that of the human eye. Short and long term color variations in an installation employing my system are less than those discernible by the human eye. My system can be used reliably over the entire spectrum of color and does not require recalibration when major grade changes are made. It permits of decreased use of dyes and better control of costly fillers in the manufacture of a colored web such as paper or the like. My system puts out signals which are specially adapted for use in a digital computer for efficient on-line control to desired standards.